It is known that isobutylene and isoamylenes, and other isoalkenes or isoolefins, produced by hydrocarbon cracking may be reacted with methanol and other C.sub.2 -C.sub.4 lower aliphatic alcohols, or alkanol, over an acidic catalyst to provide methyl tertiary butyl ether (MTBE), methyl tertiary-amyl ether (TAME) or the like. Generally, it is known that asymmetrical ethers having the formula (CH.sub.3).sub.3 C--O--R, where R is a C.sub.1 -C.sub.4 alkyl radical, are particularly useful as octane improvers for liquid fuels, especially gasoline.
MTBE, ethyl t-butyl ether (ETBE), tert-amyl methyl ether (TAME) and isopropyl t-butyl ether (IPTBE) are known to be high octane ethers. The article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The octane blending number of MTBE when 10% is added to a base fuel (R+O=91) is about 120. For a fuel with a low motor rating (M+O=83) octane, the blending value of MTBE at the 10% level is about 103. On the other hand, for an (R+O) of 95 octane fuel, the blending value of 10% MTBE is about 114.
The liquid phase reaction of methanol with isobutylene and isoamylenes at moderate conditions with a resin catalyst is known technology, as provided by R. W. Reynolds, et al., The Oil and Gas Journal, Jun. 16, 1975, and S. Pecci and T. Floris, Hydrocarbon Processing, December 1977. An article entitled "MTBE and TAME--A Good Octane Boosting Combo," by J. D. Chase, et al., The Oil and Gas Journal, Apr. 9, 1979, pages 149-152, discusses the technology. Preferred catalysts are polymeric sulfonic acid exchange resin such as Amberlyst 15 and zeolites such as zeolite Beta and ZSM-5. The acid resin catalysts are effective catalysts at temperatures below 90.degree. C. At higher temperatures the resin catalyst is unstable. Typically, with acid resin catalyst the etherification reaction is carried out in liquid phase. However, mixed phase etherification is known, particularly where the catalyst is contained as a fixed bed in a fractionator which serves to both separate the reaction products and operate as a vessel to contain the catalyst under etherification conditions. Smith U.S. Pat. No 4,978,807 describes an etherification catalyst reaction zone contained within a distillation tower.
European Patent Applications 0055045 and 0045618 describe isoolefin etherification catalyzed by zeolite. Chu et al. U.S. Pat. No. 4,605,787, incorporated herein by reference, describes a process for the preparation of methyl tertiary butyl ether which comprises reacting isobutylene and methanol in the presence of zeolite catalyst.
Typical hydrocarbon feedstock materials for etherification reactions include olefinic streams, such as cracking process light gas containing butene isomers in mixture with substantial amounts of paraffins including n-butane and isobutane. The C.sub.4 components usually contain a major amount of unsaturated compounds, such as 10-40% isobutylene, 20-55% linear butenes, and small amounts of butadiene. Also, C.sub.4 + heavier olefinic hydrocarbon streams may be used, particularly mixtures of isobutylene and isoamylene. C.sub.5 + olefinic hydrocarbon streams containing isoamylene comprising fluid catalytic cracking (FCC) gasoline are an especially important feedstock.
Improvements in the etherification of isoamylenes to TAME in C.sub.5 + FCC gasoline are very desirable to meet the amended requirements of the Clean Air Act with respect to gasoline oxygen content while avoiding C.sub.4 - hydrocarbon evaporative emissions. These amendments specify that gasoline sold in CO non-attainment areas during winter months will have 2.7 wt % oxygen by 1992 while in ozone non-attainment areas 2.0 wt % year round must be achieved by 1995.
As noted above, the use of zeolite catalyst for the etherification reaction of lower alkanol with isoolefins to produce MTBE and/or TAME is well known in the art. Among the advantages in employing acidic zeolite for the catalysis of etherification is the fact that it is much more readily regenerable than acidic resin catalyst. While sulfonated resin catalysts such as Amberlyst-15 are highly effective as etherification catalysts the fact that they are organic resins limits the temperatures to which they can be exposed without degradation. Zeolites, on the other hand, are stable at high temperatures which allows the repeated regeneration of deactivated catalyst. High temperature catalyst regeneration is by far the preferred route for regeneration to remove carbonaceous deposits, particularly produced by diene contaminates in the etherification hydrocarbon feedstream.
It is known that conjugated dienes such as butadiene and higher C.sub.5 + conjugated dienes readily oligomerize on the surface of acidic catalyst particles at moderate temperature forming carbonaceous deposits which accelerate catalyst deactivation. This mode of acidic solid catalyst particle deactivation is relatively generic to olefin conversion processes, particularly zeolite catalyzed etherification of olefinic gasoline streams. Consequently, pretreatment of the feedstock by hydrotreating and stripping is conventionally carried out to lower the concentration of diene in the feedstock. Basic nitrogen compounds, i.e., ammonia or amines, are also known catalyst poisons in etherification processes where they react with acid catalyst sites and lower catalyst activity. To avoid this problem, water washing of the feedstock is usually prescribed. Overall, the pretreatment of etherification feedstock to lower the rate of catalyst deactivation caused by dienes and basic nitrogen compounds represents a substantial economic burden on the etherification process.
It is an object of the present invention to provide an improved process for the removal of catalyst poisons comprising conjugated dienes, thiophenic sulfur and/or basic nitrogen compounds from olefinic hydrocarbon feedsteams used in zeolite catalyzed olefin conversion processes.
It is a particular object of the invention to provide a process for removing conjugated dienes, thiophenic sulfur and/or basic nitrogen compounds from olefinic hydrocarbon feedstreams employed in solid acid catalyzed etherification processes for the production of alkyl tertiary alkyl ethers.
Yet a further object of the invention is to provide a process for removing catalyst deactivating amounts of dienes from solid acid catalyzed olefin etherification process feedstreams that are carried out on C.sub.4 + or C.sub.5 + olefinic gasoline boiling range hydrocarbons.